Searching for Light Relics with Large-Scale Structure

TL;DR

Future large-scale structure surveys combined with CMB measurements can significantly improve constraints on light relics like neutrinos, especially through analyzing the galaxy power spectrum shape and BAO phase shifts.

Contribution

This paper demonstrates how upcoming large-scale structure data can enhance the detection of light relics beyond current CMB-only constraints.

Findings

Power spectrum shape measurements improve relic constraints.

BAO phase shift detection at high significance.

Enhanced sensitivity to $N_ ext{eff}$ with combined data.

Abstract

Light thermal relics of the hot big bang, often quantified by the parameter $N_\mathrm{eff}$, are one of the primary targets of cosmological measurements. At present, the energy density in such relics is constrained to be less than ten percent of the total energy density in radiation. Upcoming cosmic microwave background (CMB) experiments, however, have the potential to measure the radiation density at the one-percent level, which is close to well-motivated theoretical targets. In this paper, we explore to what degree the CMB observations can be enhanced by future large-scale structure surveys. We carefully isolate the information encoded in the shape of the galaxy power spectrum and in the spectrum of baryon acoustic oscillations (BAO). We find that measurements of the shape of the power spectrum can significantly improve on current and near-term CMB experiments. We also show that the phase shift of the BAO spectrum induced by relic neutrinos can be detected at high significance in future experiments.